For many years the primary material used to control water hardness in detergent products has been sodium triphosphate at levels of approximately 50% by weight of the finished detergent product. Within the past few years the use of high levels of sodium triphosphate has come under scrutiny because of the suspicion that soluble phosphate species accelerate the eutrophication or aging process of water bodies. This eutrophication is ordinarily evidenced by the rapid growth of algae in the water body.
Those skilled in the detergency arts have for many years equated the alkali metal pyrophosphates such as sodium pyrophosphate Na.sub.4 P.sub.2 O.sub.7 with the alkali metal triphosphates such as sodium triphosphate Na.sub.5 P.sub.3 O.sub.10 from the standpoint of builder effectiveness. It is known that pyrophosphates and triphosphates when used as builders will each sequester one mol of calcium or magnesium water hardness per mol of the phosphate species. Pyrophosphate detergency builders however, unlike triphosphates, will under certain conditions form minor amounts of dicalcium pyrophosphate, a completely neutralized insoluble salt. The precipitation of the pyrophosphates discussed above takes place on the fabrics being cleansed or upon the surfaces of the washing machine. Such buildup of the calcium pyrophosphate sale is undesirable in that the precipitates have a tendency to remain on the fabrics through several wash cycles and continue to buildup, thus causing harsh fabric. Similarly, the calcium pyrophosphate will after several cycles in a washing machine cause unsightly deposition or scale on the exposed surfaces. This precipitation phenomenon of the pyrophosphates has led many detergent manufacturers to avoid pyrophosphates and to instead use triphosphates.
It was suggested in U.S. Pat. No. 2,381,960 to Johnson, issued Aug. 14, 1945, that the effectiveness of sodium pyrophosphate in reducing water hardness could be obtained by adding the pyrophosphate to the solution of hard water after an alkaline material had been first added to the water. The alkaline materials taught by Johnson were sodium orthophosphate, sodium and potassium hydroxide, sodium and potassium carbonates and sesquicarbonates, soap, and sodium silicate having an SiO.sub.2 :Na.sub.2 O weight ratio greater than 1.5. The point of addition of the tetrasodium pyrophosphate to the solution is stated to be prior to the formation of a macroscopic precipitate of the hardness ions and the aforementioned alkaline materials.
In U.S. Pat. No. 2,326,950 issued Aug. 17, 1943 to Kepfer, it is taught that tetrasodium pyrophosphate is used more efficiently to control water hardness when it is used in conjunction with an alkaline material which added to the solution before a macroscopic precipitate of the hardness ions and the pyrophosphate is formed. The alkaline materials taught by Kepfer are sodium borate, sodium metasilicate (SiO.sub.2 :Na.sub.2 O:1:1) and disodium hydrogen orthophosphate. The processes of controlling water hardness by somehow increasing the effectiveness of sodium pyrophosphate taught by Kepfer and Johnson, herein incorporated by reference, suffer from the defect that the composition could not be effectively prepared without going to the trouble of positively insuring the delay of the pyrophosphate relative to the alkaline materials or vice versa. Thus the inventions of Kepfer and Johnson could only with great difficulty be adapted for use in granular or liquid products which are favored by the consumer today.
It can be seen, however, that pyrophosphate has the potential of being a much more effective detergency builder than the triphosphate salts. In the first instance it has a molecular weight advantage which allows more mols of the pyrophosphate salt to be present in a composition at a given weight fraction of elemental phosphorus than does triphosphate.
More importantly, the pyrophosphate tetravalent anion has a high association constant with the first calcium ion with which it associates. This first association product is the monocalcium pyrophosphate divalent anion. The divalent anion has a very small association constant with the second calcium ion to form the dicalcium pyrophosphate salt which is electrically neutral. In the absence of any material which makes the dicalcium pyrophosphate more stable, one of the associated calcium ions will be free to disassociate and to seek a more stable association such as with body soil on the fabrics or with anionic detergent. The main purpose of controlling calcium ions, whether free or associated in a weak complex, is to prevent the lastmentioned reaction with the detergent or soiled fabric from occurring. As the pyrophosphate anion strongly holds one mol of calcium ion per pyrophosphate anion it has been common practice to attempt the first association (sequestration) on a mol for mol basis. If, however, the pyrophosphate anion can be induced to strongly associate (precipitate) with two mols of calcium ion the pyrophosphate level used could be reduced substantially.
British Pat. No. 943,405, invented by Store et al, assigned to the Co-Operative Wholesale Society, Limited, and published Dec. 4, 1963 discloses a soap composition containing sodium metasilicate pretreated with fatty acid. The alkalinity of the metasilicate having a molar Na.sub.2 O/SiO.sub.2 ratio of 1/1 is treated with 88-90% of the stoichiometric amount of fatty acid based on the alkalinity of the silicate, and the resultant material is incorporated into soap powder which contained no synthetic surfactant and no inorganic builder.
U.S. Pat. No. 3,708,428 issued Jan. 2, 1973, to McDonald discusses the reacting of anionic surfactant-forming acids such as fatty acid and alkyl benzene sulfonic acid with sodium silicates having an Na.sub.2 O/SiO.sub.2 weight ratio of from about 2:1 to about 1:4 to form an in situ colloidal "silica" sol having an Na.sub.2 O:SiO.sub.2 weight ratio of from about 1:4 to about 1:2000 or greater. The reactants are said to be used in approximately stoichiometric proportions.
Detergent compositions containing this colloidal silica are said to have enhanced cleaning ability. However it was "important" to McDonald that the inclusion of water soluble alkali builders, illustrated by sodium or potassium orthophosphates, or complex phosphates (i.e. triphosphates, pyrophosphates, and glassy phosphates), alkali carbonates, borates, silicates, or neutral salts, such as sodium chloride or sodium sulfate, or magnesium sulfate, be restricted to levels of concentration of the order of 4% by weight or lower. McDonald goes on to state that the desired advantages of the in situ formed colloidal silicas on the removal of soils will be found to be appreciably impaired if the foregoing limits are not strictly adhered to.
Soluble silicates are often added to detergent or additive products to protect exposed machine surfaces from corrosion. It has been found, however, that soluble silicates interfere with the precipitation of calcium hardness as dicalcium pyrophosphate. Surfactants commonly used in detergent products have also been found to interfere with dicalcium pyrophosphate precipitation.
It has now been found that the pretreatment of silicate with acid prior to its incorporation into the detergent product diminishes the tendency of soluble silicates and surfactants to interfere with the precipitation of calcium by pyrophosphate. This effect was not only unrecognized by McDonald; it is contrary to the clear import of McDonald.
The use of alkali metal pyrophosphate in conjunction with the acidified silicate of the present invention in a detergent composition substantially decreases the amount of calcium ion in the wash solution. While not wishing to be bound by theory, it is believed that acidified silicate encourages the precipitation of calcium ions in the wash solution as insoluble dicalcium pyrophosphate. Thus, while in prior art compositions pyrophosphate salts were only able to effectively sequester on a 1:1 mol basis with calcium hardness, the present invention allows the pyrophosphate to effectively control twice the amount of calcium which is possible by sequestration. The precipitation of the dicalcium pyrophosphate takes place with the aid of the acidified silicate such that the precipitated salt does not form an unsightly scum on the exposed surfaces of the washing machine or deposit on fabrics to cause a harsh feel.
Objects of the present invention are to more efficiently utilize alkali metal pyrophosphates as detergent builders; to provide a reduced phosphorus content detergent product without substantially impairing cleaning in hard water; to utilize an alkali metal pyrophosphate salt in such a manner that it precipitates as the dicalcium salt thereof rather than sequesters calcium; to provide greater cleaning due to increased hardness control in a product of limited phosphate content; and to diminish the deposition of water hardness salts upon fabrics in the wash solution.
These and other objects of the invention which will become apparent can be achieved by preparing a detergent composition comprising alkali metal pyrophosphate, acidified silicate and surfactant.